The NYPD began fitting police helicopters with heavy machine guns to shoot down terrorist airplanes about five years ago, after intelligence surfaced that al-Qaida was considering the use of crop dusters and other small craft for attacks, a senior city law enforcement official said Monday.

But while police officers have been trained to use helicopter-mounted .50-caliber machine guns against light planes, there was no intention to take down commercial passenger jets with such weaponry, said the official, who asked not to be identified.

"It is not surface-to-air missiles or a weapon that has rockets," said the official, who declined to say if helicopters armed with the machine guns are kept on standby in case they are suddenly needed.

On Sunday, during a "60 Minutes" interview, NYPD Commissioner Ray Kelly said that in "an extreme situation" cops have the ability to take down an aircraft that is threatening the city.

Monday, a spokesman for the North American Aerospace Defense Command, the key military unit defending the nation's airspace, indicated that the city could take action in an immediate local threat.

"Each state and city has the right to execute law enforcement responsibilities, while NORAD remains responsible for defending the United States and Canada from air threats that are considered an attack on our nations," said James Graybeal, director of public affairs for NORAD.

One example was the response by Austin, Texas, police in 2010, when a local man flew a light plane into an IRS building, killing himself and damaging the building, one NORAD official said.

Kelly didn't want to give specifics in the interview about the police weapons. But the law enforcement official said the focus was on the .50-caliber machine guns, which have been in use for decades, and not the more rapid-fire weapons capable of throwing out thousands of rounds a minute.

"A well placed shot from .50-caliber could take down a big airplane or small airplane," said Dr. Robert Latiff, a professor of national security and intelligence at George Mason University in Fairfax, Va., and a retired Air Force general. "An incendiary round, that would make a big difference."

Kelly said in his interview that after 9/11, the city realized it couldn't rely on the federal government alone to fight terrorism. He hinted that police would try to intercept a terrorist aircraft only in a last-ditch situation.

"If you are facing a potential death blow, what are you going to do?" noted Latiff.